The powerhead runs on AC.
A dimmer will not work. Dimmer switches are designed to dim resistive loads... like light bulbs, not inductive loads, which are what motors are.
Using a dimmer will probably not hurt the powerhead, but it won't work well if at all. The reason for this is that this type of motor's speed is dependant on two things... A) the design of the motor (whcih you can't change obviously), and B) the frequency (not voltage) of the power supply.
What you would need to really do this is something called a VFD, or Variable Frequency Drive. These are found in the industrial world, to control the speed of machinery. They are crazy expensive, and usually very large, though they do make them for smaller motors.
An alternative that I might say is worth a shot is the speed control that is specifically designed for ceiling fans. These units usually have 4 or so speeds, as opposed to a variable control. It may work, though I've never tried it. They use capacitors to change the power factor, which works better on motors than what a dimmer switch does. Be sure to get one that is designed for ceiling fans, and not an ordinary dimmer.
Lemme know how it works if you try it.
